xfs: split xfs_iformat_fork

xfs_iformat_fork is a weird catchall.  Split it into one helper for
the data fork and one for the attr fork, and then call both helper
as well as the COW fork initialization from xfs_inode_from_disk.  Order
the COW fork initialization after the attr fork initialization given
that it can't fail to simplify the error handling.

Note that the newly split helpers are moved down the file in
xfs_inode_fork.c to avoid the need for forward declarations.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>